You are not legally obliged to tell your employer you are pregnant until 15 weeks before your due date. However, you may need a risk assessment and so will need to judge when you tell your employer.
No one can tell you how your manager/employer will react. Good employers will have encountered this before and will have policies in place. Poor employers can and do act unlawfully. Pregnancy and maternity are protected characteristics and if you are dismissed due to being pregnant you can claim unfair dismissal, even though you have only been there a few weeks. ACAS and Pregnantthenscrewed website can help, if you need it.
Your key concern should be checking your eligibility to Statutory Maternity Pay www.gov.uk/maternity-paternity-pay-leave
Then look at what benefits, if any, you can get whilst on mat leave. CSAB
You are in a good position in terms of LTV and so may be able to work with your mortgage company to look at ways to reduce your monthly mortgage, if necessary, whilst on mat leave e.g. extending term, mortgage holiday.
You have savings which is good, look at Money Saving Expert to help you draw up a realistic budget and identify savings. This will help you work out how much mat leave you can afford to take. Buying new for a baby is lovely but realistically some items are hardly used and so you can get some good second hand items. Baby forums will tell you where you should spend (car seat) and where you can cut costs by having second hand.
Career wise, it will depend on the profession, but there is no reason you can’t continue to build your career. Remember, your child will be 10 when you are 40, and you will have 20+ years to retirement.
I have deliberately not mentioned financial assistance from the father, as whilst this may be forthcoming the reality is many try not to pay it.
I’ve given you a lot to think about, and I’m sure others can add to this, please don’t worry you are not the first to be in this situation and you seem better placed than many to cope.
Congratulations 
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